As catalysts for reducing and then converting NOx that are included in exhaust gases from lean-burn engines of automobile, the following have been known: lean NOx catalysts, NOx storage-and-reduction catalysts, NH3 denitration catalysts, NOx selective reduction catalysts, and the like. Of these, lean NOx catalysts are used mainly in exhaust gases of diesel engine; and they reduce and then convert NOx by means of reducing agent, such as light oil, which is added into the exhaust gases.
The NOx storage-and-reduction catalysts use NOx storage materials, such as alkali metals and alkaline-earth metals; have the NOx storage materials store NOx in lean atmospheres; and thereafter the ambient atmospheres are turned into rich atmospheres, thereby reducing and then converting NOx being released from the NOx storage materials by means of reducing components that exist abundantly in the ambient atmospheres.
Moreover, the NH3 denitration catalysts reduce NOx by means of NH3, which is generated by adding urea water and the like into exhaust gases, as set forth in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 10-146,528, for instance.
However, in the lean NOx catalysts and NOx storage-and-reduction catalysts, there might be such a problem that NOx have been discharged as they are because it is difficult to reduce NOx in regions where the temperature is at around or lower than about 250° C. at which supported noble metals, such as Pt, are activated. Moreover, in the NH3 denitration catalysts as well, lowering the activated temperature has been carried out by using noble metals like Pd combinedly, because the temperature at which NH3 and NOx react with each other is high originally. However, even in this case, it is difficult to convert NOx up to the activation temperature at which the noble metals are activated, similarly to the lean NOx catalysts and NOx storage-and-reduction catalysts.
Hence, in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2000-230,414, the following are proposed: putting an NOx adsorption material in place on the exhaust-gas upstream side of an NOx reduction catalyst that comprises a lean NOx catalyst or NH3 denitration catalyst. In accordance with such an exhaust-gas converting apparatus, NOx are adsorbed onto the NOx adsorption material in low-temperature region, and thereafter NOx, which have been released from the NOx adsorption material, are reduced and then converted by the downstream-side NOx reduction catalyst in high-temperature region. Therefore, it is possible to inhibit the discharge of NOx from low-temperature region and up to high-temperature region.
As such an NOx adsorption material, one in which Pt is supported on alumina is exemplified in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2000-230,414, and absorbing NOx at such temperatures as up to about 230° C. is set forth therein. Moreover, an NOx adsorption material, which comprises zeolite in which at least one member being selected from the group consisting of Fe, Cu and Co is supported by means of ion exchange, is set forth in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2007-160,168. In addition, an NOx adsorption material, which comprises at least one oxide that is selected from the group consisting of Co, Fe and Ni, is set forth in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2001-198,455, and adsorbing NOx in low-temperature region of 40° C. or less is set forth therein.
Furthermore, in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2007-160,168, the following are set forth: the zeolite on which at least one member that is selected from the group consisting of Fe, Cu and Co is supported by means of ion exchange demonstrates high NOx adsorbing capability from such ordinary temperature as room temperature approximately.
Incidentally, in a case where an NOx adsorption material and an NOx reduction catalyst are put in place in the exhaust-gas flow passage of automobile, the NOx adsorption material and NOx reduction catalyst are accommodated within a cylinder-shaped converter. However, within the cylinder-shaped converter, a temperature distribution occurs in which the axially central section becomes higher temperatures and the outer peripheral section becomes lower temperatures. Moreover, in general, the temperature is higher on the more upstream side, and the temperature becomes lower on the more downstream side.
Consequently, in a case where an NOx adsorption material exhibiting comparatively low temperatures at which NOx are eliminated therefrom is used, NOx that are released from the NOx adsorption material have been discharged as they are when the downstream-side NOx reduction catalyst does not reach up to the activation temperature. Moreover, in a case where an NOx adsorption material exhibiting comparatively high temperatures at which NOx are eliminated therefrom is used, there might be such an instance that the amount of NOx flowing into the NOx reduction catalyst becomes an excessive amount that surpasses the ability of the NOx reduction catalyst, because no NOx can be adsorbed after the NOx adsorption amount of the NOx adsorption material has been saturated; if such is the case, an amount of get-past NOx, which have been discharged without being reduced by the NOx reduction catalyst, has become greater.    Patent Literature No. 1: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 10-146,528;    Patent Literature No. 2: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2000-230,414;    Patent Literature No. 3: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2001-198,455; and    Patent Literature No. 4: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2007-160,168